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Some weeks ago I installed the owncloud client on my Ubuntu system. However, it does not seem to be safe and I want to remove it. I deleted it and apparently it is gone. Unfortunately there is still an persistent entry in the software update. Each time I want to update my system an error aborts everything, because I did not import the public key:

sudo apt-get update
Reading package lists... Done
W: GPG error:  Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 977C43A8BA684223

I just could import the public key and no error message would annoy me any longer. But I don't want to trust this key, but simply remove everything concerning owncloud.

I've opened /etc/apt/sources.list, but I cannot find Why? Where does apt-get get his source list? How can I solve this problem?

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Did you use the add-apt-repository command to add the repository? – Arronical Jan 5 at 9:57
Did you check the files in /etc/apt/sources.list.d/??? – wb9688 Jan 5 at 11:44
No, I didn't. In fact there are files that seem to belong to owncloud. What is this folder about? – null Jan 8 at 10:22
up vote 3 down vote accepted

If you used the add-apt-repository command, or its alias apt-add-repository, the PPA or source repository can be found in the /etc/apt/sources.list.d directory.

It can be removed correctly using:

add-apt-repository -r

I'm not aware of any bad effects from just deleting it from the /etc/apt/sources.list.d directory, but I can't say for certain that there will not be any.

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You can just delete the files. It works like a charm. – null Jan 8 at 10:34
  1. Go to Unity dash and search for Software Sources

    enter image description here

  2. Go to Other Software and select the desired PPA to be removed.

  3. Click Remove

    enter image description here

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